Microwave communications systems require filters with sharp frequency selectivity, flat in-band slope and small group delay. Many applications, particularly for satellite systems, necessitate that these characteristics be realized in a device of minimum weight and volume. These requirements have been met by constructing narrow bandpass waveguide filters employing multiple-coupled cavities.
A number of filter structures have been proposed in which circular cylindrical cavities are utilized which are resonant in the TE.sub.111 mode or the TE.sub.011 mode. One such structure is disclosed in U.S. Pat. No. 3,969,692. A number of these cavities are used in each filter structure and are either electrically or magnetically coupled by suitable apertures at contiguous points between cavities. TE.sub.011 mode filters generally have higher Q's (lower losses) than TE.sub.111 mode filters, but this advantage is offset by the somewhat larger size requirements and more restricted frequency band in which no other mode is excited. Further, the TM.sub.111 mode is degenerate with the TE.sub.011 mode, i.e. any cavity designed to support TE.sub.011 resonance will also be capable of supporting TM.sub.111 resonance. Thus, the TM.sub.111 resonance must be suppressed in some way.
A disadvantage which is characteristic of both TE.sub.111 and TE.sub.011 mode filters is that, in cases where a multicavity filter is required, for example N cavities, the filter structure must include N separate cylindrical cavities so that the resulting filter structure has a volume which is N times the volume of a single cavity and a weight which is also increased proportionally.
This drawback is somewhat alleviated by the use of dual mode filter structures, such as that described in application Ser. No. 754,804, now U.S. Pat. No. 4,060,779 filed Dec. 27, 1976 and assigned to the same assignee as the present invention. However, even in the case of such dual mode filters, an N-cavity electrical filter will still require N/2 physical cavities. Thus, there is a need for a more compact N-cavity filter structure.